Process and apparatus for wet scrub removal of dust and mist from gases



Feb.. 27, 1968 Filed Feb. 1, 19

M (ORIFICE- VENTURI) '0? Lu 5 Q G Lug E LL ll go w (i 2 \4 U1 0 Z 7 CL 2 V R. 1.. LUCAS ETAL PROCESS AND APPARATUS FOR WET SCRUB REMOVAL OF DUST AND MIST FROM GASES 67 POLYETHYLENE FIBROUS BED 5 Sheets-Sheet 1 l0 PRESSURE DROP-IN. WATER F IG. 2

I6 I lo GAS T I Fl BROUS BED STRUCTURE FIG.I

INVENTORS HAROLD F. PORTER ROBERT L.LUCAS p owu ATTORNEY Feb. 27, 1968 PROCESS AND APPARATUS FOR WET SCRUB REMOVAL Filed Feb. 1, 1967 R. L. LUCAS ETAL OF DUST AND MIST FROM GASES 3 Sheets-Sheet 3 10,000 3.5 MIL. Fl BER 99' 99 DIAMETER 6MlL.FlBER a? DIAMETER a 2 E U m 0 LL 5 g E 5 F100 99D 2 J o z m U g g 3 8 8 6 6 5 U FIBROUS BED 5' E w \ISEFFICIENCY FOR 0 2 MICRON PARTICLES C lo PM 90.0 1 0:

Lo I l O THICKNESS OF Fl BROUS INVENTORS BED STRUCTURE- INCHES HAROLD F. PORTER ROBERT L. LUCAS m Q- GMZ ATTORNEY United States Patent 3,370,401 PROCESS AND APPARATUS FOR WET SCRUB REMGVAL 0F DUST AND lVHST FROM GASES Robert L. Lucas, Newark, and Harold F. Porter, Hockessin, Del., assignors to E. I. du Pont de Nemours and Company, Wilmington, Del., a corporation of Delawme Continuation-impart of application Ser. No. 371,261, June 1, 1964. This application Feb. 1, 1967, Ser. No. 613,324

2 Claims. (Cl. 55-90) ABSTRACT OF THE DISCLOSURE Process and apparatus for web scrub removal of dust and mist from gases in which the polluted gas is sprayed with a liquid in which the dust is relatively insoluble, the gas and liquid are then passed at a velocity in excess of the flooding velocity through a fibrous bed structure of pore size sufficiently fine and length of path sufiiciently long to thoroughly contact the dust and mist withthe liquid, and the dust and mist are separated as a liquid slurry, while the cleansed gas is withdrawn as a separate fraction.

Cross reference to related applications This application is a continuation-in-part of US. application Ser. No. 371,261 filed June 1, 1964, now abandoned in the names of the same applicants as joint inventors.

Brief summary of the invention Generally, this invention comprises a method of removing fine particulate solids and mist entrained in a gas stream comprising, in sequence, spraying the gas stream with a liquid in which said particulate solids are relatively insoluble, passing the gas-liquid mixture at a velocity in excess of the flooding velocity through a fibrous bed structure having a pore size sufiiciently fine and a length of path sufficiently long to thoroughly contact the gas stream containing the particulate solids and mist with the liquid, and separating as a first fraction a slurry of particulate solids containing the mist in the liquid and as a second fraction cleansed gas substantially free of solids, mist and liquid droplets, together with apparatus for carrying out the method.

Drawings The construction of apparatus according to the invention and the manner in which it operates are detailed in the drawings, in which:

FIG. 1 is a schematic sectional side elevation of a preferred apparatus embodiment,

FIG. 2 is a comparative log-log plot of Per Cent Dust Loss versus Pressure Drop in Inches of Water for this invention and three existing methods of the prior art,

FIG. 3 is a log-log plot of flooding velocity as a function of the independent variables determinative thereof, and

FIG. 4 is a semi-log plot of'Fibrous Bed Structure Thickness vs. Dust Collection Efficiency for fibrous bed structures made up of 3.5 mil and 6 mil thick fibers, respectively.

Detailed description Referring to FIG. 1, a preferred embodiment of apparatus comprises a vertical cylindrical housing 10 pro- 3,370,401 Patented Feb. 27, 1968 ice vided with a concentrically disposed gas supply conduit 11 carrying within its discharge end a plug of fibrous bed structure 12 disposed transverse the conduit. At least two conduit diameters upstream from the fibrous bed structure 12 is a concentrically disposed liquid spray nozzle 15 oriented to discharge its liquid substantially evenly over the gas passed through conduit 11. Disposed approximately one conduit 11 radius above the gas exit side of fibrous bed structure 12 is an inverted cylindrical cap 16 having a depth approximately one conduit 11 diameter, so that its rim overhangs the end of conduit 11, with its emplaced fibrous bed structure, approximately one conduit 11 radius.

The upper end of housing 10 is provided with the cleansed gas exit port 18, whereas offside port 19 in the bottom of housing 10 is reserved to solid-liquid slurry withdrawal.

The following relative proportions of equipment components have proved especially effective, it being understood, however, that these constitute no critical limitations and that individual dimensions can in fact be varied widely without affecting the operation deleteriously: Where D diameter of conduit 11, housing 10 can have a diameter of 2.51) and a height from lower dished head juncture with the cylindrical side wall to upper dished head juncture of 3.0D, with the opening of conduit 11 disposed at a level of 2.0D above the lower dished head juncture.

It is important that there be even distribution of scrubbing liquid across the entire cross-section of conduit 11, and this can be readily achieved by using one or more solid cone spray nozzles 15 of the non-clogging type.

Entrained solids and mist removal according to this invention depends upon contacting the entraining gas stream intimately with a liquid and, thereafter, collecting the liquid-solid slurry from the effluent gas stream. Traverse of a relatively fine fibrous bed structure is essential to intimate liquid-solid contacting, especially in an accompanying gas stream, however, rapid clogging of the meshes with solids occurs unless the .gas velocity is kept well above the flooding velocity for the particular fibrous bedsolids-liquid system utilized. The problem of clogging is particularly serious where, as with this invention, the particulate solids to be separated are relatively insoluble in the scrubbing liquid.

Fibrous bed structures of a great variety of constructions are suitable for the purposes of this invention; however, knitted structures are particularly preferred, because of the uniformity of flow characteristics which it is possible to readily obtain by this fabrication. Similarly, the materials of construction are of wide choice, thereby making it possible to employ those which have exceptional resistance to chemical corrosion, or erosion by the gases, liquids and solids in process.

For example, S.A.E. Type 304 stainless steel, polyethylene and polytetrafiuoroethylene have all given good results as fibrous bed structure materials.

The knitting practice can be entirely conventional, the cardigan stitch being preferred, but other stitches, such as the half-cardigan, cable and the like being equally suitable. Overall fiber diameters ranging from 0.5 mil to 16 mils, or even thicker, proved satisfactory, both as monofilaments and also as multifilaments. The prefer-red range of mesh sizes for the knitted material was quite broad, extending from about $4 to about A".

Both circular and flat bed knitting machines can be employed, the circular type product, which is sleeve-like in form as received from the knitting machine, being preferably flattened to double thickness and then crimped along diagonal lines /4" apart to bond the two thicknesses at, typically, spacings, thereby imparting a somewhat bulked, quilt-like freeness normal to the thickness of the knitted material, which resists excessive compaction under the impact of the liquidgas stream in transit therethrough.

It is convenient to make up circularly shaped fibrous bed structure by simply winding a length of the knitted material upon itself in a continuous spiral, and then cutting it to the thickness desired for plug 11 (typically 4"), whereupon the direction of gas liquid flow therethrough is end-on, or parallel to the axis of roll-up. Alternatively, a number of layers of the knitted material may be .cut to any desired common size and shape and stacked in a pile to make up the fibrous bed structure, both the polymeric materials and the metals being sufficiently stiff :so that the double thickness knit goods springs apart to give an open fabrication with relatively low resistance to gas throughput, as will be understood when it is considered that only thirty-six double layers of 4 mil polyethylene has an uncompressed thickness of approximately 4". v V Because of the variety of constructions and'random layer-to-layerplacements which fibrous bed structures can take, the most reliable indices for comparison are believed to be the total calculated specific surface of the strand material utilized in making up a specific volume,

the packing density and the fraction of voids present in a cross-section opposed to the gas-liquid flow. The following is a tabulation of these characteristics for typical knitted structures which have given good service in dust separation tests: 7

Strand Bolypropylene g In general, fibrous bed structures which have a packing density of about 1-20 lbs./ft. a void fraction of about 80-99% and a specific surface of about 50 ftF/ftfi or greater give good dust separation etficiency. The specific fiber arrangement appears to be of secondary importance and, thus, knitted structures, random packed fibers, woven fabrics, multiple layers of screens, or combinations of two or more of any of these are all satisfactory for the purposes.

In operation, scrubbing liquid supply rates can be varied overfrelatively wide ranges, rates increasing from 3 g.p.m./ft. to 60 g.p.m./ft. of fibrous bed structure cross-sectional area having no appreciable eifect on particulate solids separation efiiciency, which remained very high. On the basis of gas throughputs, apparatus of this invention operated successfully at scrubbing liquid supply rates of 1.0'gal./ 1000 ft. of gas to 100 gals./ 1000 ft. of gas containing grains of dust of the general analysis hereinafter detailed per cubic foot of entraining gas. At higher liquid supply rates, dust loadings in excess of 200 grains/ft. were satisfactorily scrubbed out.

The scrubbing liquid can be supplied either fresh or recycled repeatedly through the apparatus, it being preferred to use the recycled slurry, because it is thereby possible to obtain a concentrated fraction of recovered solids for either return to the process, m'arketingin slurry form, or more convenient disposal as waste by virtue of its concentrated volume. The ultimate concentration continued scrubbing with slurries containing as high as 12% solids content was accompanied by no difliculties. In fact, comparative tests shown that the slurry appeared to have a slightly higher solids scrub removal efliciency than clear liquid.

It is essential that the gas-liquid throughput'past the: fibrous bed structure 12 be in excess of the flooding velocity, which is defined by T. K. Sherwood and associates in Industrial and Engineering Chemistry, vol. 30 (1938),. pages 765-769. Flooding entails the more or less complete and vigorous rapid expulsion of liquid from the filter structure along with the gas throughput. If the flooding velocity is not exceeded, solids will collect in the interstices, rapidly blinding the bed structure to further gasliquid passage. 7

FIG. 3 constitutes a convenient plot of the flooding velocity in terms of v zs tuz c plr as abscissa against as ordinate where:

From the foregoing, it will be seen that the flooding velocity depends upon the characteristics of the fibrous bed structure employed and also on the materials being processed, as well as the rates at which processing is effected. The relationship is a complex one but good operation is obtained whenever functioning is above the curve plot of FIG. 3, which can be assured by a few check readings of the parameters which can be varied; for example, the superficial gas velocity, followed by appropriate adjustment to bring operation into the region above the flooding velocityplot line.

The following operating tests were conducted in an apparatus employing a 4" dia. conduit 11, an exiting liquid slurry impingement cap 16 of 8" dia. and a vessel 10 dia. of 12''. The entrained particulate solid employed was water-insoluble talc dust having an average particle size of about 2 microns, of the'material being finer than 10 microns. The cumulative percentage sieve analysis showed that 50% was finer than 2.4 microns, 28% was finer than one micron and 14% was finer than 0.5 micron particle size. The standard test duration was 20 minutes;

however. Runs 46 and 51 were each of 3 hours duration.

In none of the runs was there any sign of plugging of the fibrous bed structures.

In all of the tests knitted fibrous bed structures 12 measuring 4" in dia. were employed, these being mount:

, ed transverse. gas supply conduit 11 as shown in FIG. 1,

with an overall thickness in the direction of gas flow of 4" in all instances, except as regards Runs 51 and 53,

marked with asterisks, where the thickness was 8". The.

SERIES A Runs 40-44,- den'sity of fibrous bed structure :4 lbs./

ftfi; temperature of air-dust mixture 16 C.; rate of spray liquid supply 22 g.p.m./ft. transverse conduit 11 crosssection:

Run 46-48.Rate of liquid supply, 24 g.p.m./ft. transverse conduit 11 cross-section.

Test T emp.Run 46, 12 C.; Test 47, 13 C.; Test 48, 6 C.

Superficial Gas Gas Dust Pres- Gas Velocity Contamisure Percent Run Through- Through nation of Drop, Dust 001- No. put, Fibrous Input, Inches lection itfi/min. Bed StrucgrJttfi H20 Efiiciency ture, itJsec.

Runs 51 and 53, rate of liquid supply 24 and 25 g.p.m./ ft. conduit 11 transverse cross-section, respectively; test temperature 12 C. and 15 C., respectively.

Runs 55 and 56, density of fibrous bed structure, 20 lbs./ft. test temperature 15 and 17 C., respectively; rate of liquid supply 10 g.p.rn./ft. transverse conduit 11 cross-section.

Superficial Gas Gas Dust Pres- Gas Velocity Contamisure Percent Run Through- Through nation of Drop, Dust Col- No. put, Fibrous Input, Inches lection itfi/min. Bed Strucg'r./l't. H2O Efficiency ture, ft./see.

SERIES C Runs 58 and 59: density of fibrous bed structure=40 lbs/ft?" test temperature 17 C.; rate of liquid supply, 10 g.p.m./ft. conduit 11 transverse cross-section.

Superficial Gas Gas Dust Pres- Gas Velocity Contarnisure Percent Run Through- Through nation of Drop, Dust Col- No. put, Fibrous Input, Inches lection ti/min. Bed StrucgrJit. H 0 Efliciency ture, ft./sec.

Runs 64 and 65: density of fibrous bed structure, 12 lbs./ft. test temperatures 13 C. and 16 C., respectively; rate of liquid supply 10 g.p.m./ft. conduit 11 transverse cross-section.

Superficial Gas Gas Dust Pres- Gas Velocity Contamisure Percent Run Through- Through nation of Drop, Dust Col- No. pu Fibrous Input, Inches lection ftfi/mrn Bed Strucgin/it. H20 Efiicieucy ture, ftJseo.

From the foregoing, it will be seen that an exceedingly high dust collection efficiency is attained in all cases, with an accompanying relatively low pressure drop. Moreover, the test durations were sufliciently long, i.e., 20 minutes to 3 hours, so that complete assurance against clogging was obtained.

The FIG. 3 ordinate and abscissa values corresponding to the individual Run Nos. hereinabove set forth are tabulated as follows, only the six runs lying nearest to the critical flooding velocity line being plotted as individual points identified by their respective Run Nos. on the figure:

Run Ordinate Abscissa Run Ordluate Abscissa N o. No.

Yet other tests conducted with water-insoluble titanium dioxide of less than 5 microns size as the entrained solids showed results substantially the same as for the talc.

Referring to FIG. 4, smaller diameter fibers have proved more efiicient on an equal weight per unit volume basis than larger diameter fibers, probably because the increased length of the former makes the gas-liquid path through the fibrous bed structure more serpentine. Thus, in a series of tests on air-entrained 2 micron average particle size talc, improved separating efliciencies' Were obtained per inch of fibrous bed structure thickness with 3.5 mil fiber diameter over those with 6 mil fiber diameter. The material of fabrication was ASAE 304 stainless steel in knitted form (mesh size about 7 on a side), but uncrimped, with the following characteristics:

The comparative performance of the apparatus of this invention as opposed to three conventional types of scrubber apparatus employed in particulate solids removal from gas streams is shown in FIG. 2. The same 2 micron average particle size Water-insoluble dust hereinbefore described was employed in air entrainment as the test medium, at air flows ranging from about 124 it /min. to 532 ftfi/min. (referred to 70 F., one atmosphere pressure) at dust loadings ranging from about 3.43 grains/ft. to 24.9 grains/ft. and scrubber Water supply rates ranging from 5.33 gal./ 1000 ft. of air throughput to 41.6 gal./ 1000 ft. the higher water supplies of 31 to 41.6 gal./ 1000 ft. being required to avoid plugging of the impingement plate apparatus having the performance plotted along line N. In these tests water supply rates of about 10 gal./ 1000 ft. of air were employed for the fibrous bed scrubber of this invention.

It is apparent from FIG. 2 that orifice-venturi types scrubbers with characteristics along plot M were the least eflicient in terms of dust loss of all the conventional apparatus tested. impingement plates (plot N) and packed sphere beds (5" thickness of /3 diameter marbles, plot 0) came next in order of increasing efliciency, but all were surpassed by the fibrous bed structure scrubbersof this fibrous bed structures of its accompanying water-solids slurry, including conventional cyclone separators and the like. However, water impingement against a solid plate with deflection sharply away from the general direction of gas flow through the bed structures is highly compact, economical and entirely satisfactory, so that it is preferred. Also, it is not absolutely necessary that conduit 11 'be aligned vertically, and other inclinations are practicable; however, more even scrubbing is thereby attained and vertical arrangement is therefore preferred. 7

From the foregoing, it will be understood that this invention can be modified in numerous respects without departure from its essential spirit, and it is therefore intended to be limited only Within the scope of the appended clairns.

What is claimed is:

1. A method for removing fine particulate solids and mist entrained in a gas stream comprising, insequence, spraying said gas stream with a liquid in which said particulate solids are relatively insoluble, passing the gasliquid mixture in a generally upward direction at a velocity in excess of thefiooding velocity through a fibrous bed structure having a pore size sufficiently fine and a length of path sufiic'iently long to thoroughly contact said gas stream containing said particulate solids and mist "with said liquid, and impinging effluent solid-liquid mixture exiting from said fibrous bed structure against an impervious surface redirecting the flow of a first fraction consisting of a slurry of particulate solids containing said mist in said liquid in a direction sharply away from that taken by said gas-liquid mixture through said fibrous bed structure while exhausting as a second fractioncleansed'gassubstantially free of solids by discharge in a direction in general prolongation with the course of said gas-liquid mixture through said fibrous bed structure.

2. Apparatus for removing fine particulate solids and mist entrained in a gas stream comprising, in combination,

a housing fitted at the bottom with an upwardly directed gas supply conduit discharging into said housing, said gas supply conduit being provided with a scrubbing'liquid spray nozzle dischargingaliquid in which said particulate solids are relatively insoluble substantially evenly across the cross-section of said conduit, a fibrous bed structure disposed transverse the open discharge end of said conduit fabricated from fibers having diameters in the range of about 0.5 mil to 16 mils, said fibrous bed structure having a packing density of about one to 20 lbs./ft. a specific surface in excess of about 50 fL /ft. and a void fraction in the range of about 0.8 to about 0.99, means forcing said gas stream sprayed with said scrubbing liquid through said gas supply conduit and said fibrous bed structure at a velocity in excess of the flooding velocity of said fibrous bed structure, means comprising an impervious surface disposed above said fibrous bed structure redirecting the flow of a fraction consisting of a slurry of particulate solids containing'said mist in said liquid in a direction sharply away from that taken by said gas stream and said scrubbing liquid through said fibrous bed structure and thence into a collection volume reserved for said fraction, and an open cleansed gas 'escape conduit in general prolongation with the course taken by said gas stream and said scrubbing liquid through said fibrous bed structure.

References Cited 1 SAMIH N. ZAHARNA, Primary Examiner.

J. ADEE, Assistant Examiner. 

